On Tue, Apr 26, 2016 at 08:47:32PM +0100, Chris Wilson wrote:
> On Tue, Apr 26, 2016 at 07:29:42PM +0200, Daniel Vetter wrote:
> > Finally all the core gem and a lot of drivers are entirely free of
> > dev->struct_mutex depencies, and we can start to have an entirely
> > lockless unref path.
> >
> > To make sure that no one who touches the core code accidentally breaks
> > existing drivers which still require dev->struct_mutex I've made the
> > might_lock check unconditional.
> >
> > While at it de-inline the ref/unref functions, they've become a bit
> > too big.
> >
> > v2: Make it not leak like a sieve.
> >
> > Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
> > ---
> > drivers/gpu/drm/drm_gem.c | 64 ++++++++++++++++++++++++++++++++++++++---------
> > include/drm/drmP.h | 12 ++++++++-
> > include/drm/drm_gem.h | 45 ++-------------------------------
> > 3 files changed, 65 insertions(+), 56 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
> > index 25dac31eef37..8f2eff448bb5 100644
> > --- a/drivers/gpu/drm/drm_gem.c
> > +++ b/drivers/gpu/drm/drm_gem.c
> > @@ -788,16 +788,7 @@ drm_gem_object_release(struct drm_gem_object *obj)
> > }
> > EXPORT_SYMBOL(drm_gem_object_release);
> >
> > -/**
> > - * drm_gem_object_free - free a GEM object
> > - * @kref: kref of the object to free
> > - *
> > - * Called after the last reference to the object has been lost.
> > - * Must be called holding struct_ mutex
> > - *
> > - * Frees the object
> > - */
> > -void
> > +static void
> > drm_gem_object_free(struct kref *kref)
> > {
> > struct drm_gem_object *obj =
> > @@ -806,10 +797,59 @@ drm_gem_object_free(struct kref *kref)
> >
> > WARN_ON(!mutex_is_locked(&dev->struct_mutex));
> >
> > - if (dev->driver->gem_free_object != NULL)
> > + if (dev->driver->gem_free_object_unlocked != NULL)
> > + dev->driver->gem_free_object_unlocked(obj);
> > + else if (dev->driver->gem_free_object != NULL)
> > dev->driver->gem_free_object(obj);
> > }
> > -EXPORT_SYMBOL(drm_gem_object_free);
> > +
> > +/**
> > + * drm_gem_object_unreference_unlocked - release a GEM BO reference
> > + * @obj: GEM buffer object
> > + *
> > + * This releases a reference to @obj. Callers must not hold the
> > + * dev->struct_mutex lock when calling this function.
> > + */
> > +void
> > +drm_gem_object_unreference_unlocked(struct drm_gem_object *obj)
> > +{
>
> I have i915.ko in a state where we could use this as well. I would like
> to have our inlines back though. I would add
>
> static inline void
> i915_gem_object_put(struct drm_i915_gem_object *obj)
> {
> kref_put(&obj->base.refcount, drm_gem_object_free);
> }
>
> Hmm, considering how simple that is, maybe I won't ask for
>
> static inline void
> __drm_gem_object_unreference_unlocked(struct drm_gem_object *obj)
> {
> BUG_ON(obj->dev->driver->gem_free_object == NULL);
> kref_put(&obj->refcount, drm_gem_object_free);
> }
Yeah, something like that makes sense. Imo the core versions really need
all the silly locking checks and all that to make sure no one abuses them
by accident, or breaks a driver still using struct_mutex. Adding a
fastpath for drivers using gem_free_object_unlocked just wrapping the
kref_put certainly looks like a good idea.
And if we document the exact use case I think we could even nuke the
BUG_ON too. This can be audited with a simple
$ git grep __drm_gem_object_unreference_unlocked -- drivers/gpu/drm/$driver
so imo nothing too dangerous.
Ack/r-b on the patch itself?
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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